The present invention relates generally to sample changers for X-ray diffractometers, and in particular to an automatic sample changer for use with capillary geometry powder-diffraction systems that permits high-throughput acquisition of X-ray powder patterns.
Powder X-ray diffraction techniques are used to measure the structural properties of a wide variety of materials. For a general review of X-ray diffraction, see B. E. Warren, X-ray Diffraction, Dover, 1990. Typically, a beam of X-rays is passed through a sample of randomly-oriented microcrystals to produce a pattern of rings on a distant screen. The pattern correlates with the structure of the molecules comprising the microcrystals.
Powder diffraction may be used for the identification of the structural phases in a sample which may contain multiple structural representations of a material. Powder diffraction may also be used as a means of verifying that a powder sample contains a material whose structure has been determined by a single crystal X-ray diffraction measurement. Typically, a powder X-ray diffraction pattern is calculated for a known material, after which a new measured powder pattern is compared to the known material""s pattern which is then used to verify that the sample measured is representative of the expected compound.
The development of synchrotron radiation sources has lead to advancements in powder X-ray diffraction techniques. See, e.g., Synchrotron Radiation Research, H. Winick and S. Doniach, Eds., Plenum, N.Y., (1979); Synchrotron X-ray Powder Diffraction, J. B. Hastings, W. Thomlinson, and D. E. Cox, Journal of Applied Crystallography, 17, 85(1984); and Powder Diffraction, D. E. Cox in G. S. Brown and D. E. Moncton, Eds., Handbook on Synchrotron Radiation, Elsevier, 3, 155 (1991). A synchrotron is a cyclic particle (electron, proton, heavy-ion) accelerator in which a particle is confined to its orbit by a magnetic field that gradually strengthens as the particle""s momentum increases. Acceleration is produced by an alternating electric field that is in synchronism with orbital frequency. A consequence of the acceleration is the emission of synchrotron radiation. This radiation is a broad-spectrum (white) emission that resembles the white radiation of an X-ray tube but is immensely more intense (100 to 104 times the intensity of a conventional characteristic line). See, e.g., X-ray Structure Determinationxe2x80x94A Practical Guide, pp 14-15, Stout and Jensen, 2nd edition, Wiley Interscience (1989).
There are currently several powder X-ray diffraction instruments at synchrotron radiation sources around the world where powder X-ray diffraction is practiced. The most favored geometry for carrying out powder X-ray diffraction at a synchrotron radiation source is high resolution geometry. Typically, a sample is mounted in either a flat plate or capillary geometry and X-ray beams are directed at the sample. X-rays diffracted by the sample are analyzed by an analyzer crystal before being counted by an X-ray detector. Such geometries, where the diffracted beams are analyzed by diffraction, utilize standard commercial powder X-ray diffraction instruments.
The analyzer crystal is typically referred to as a xe2x80x9cdiffracted beam monochromatorxe2x80x9d and is typically of a mosaic material such as graphite or lithium fluoride. Owing to the properties of synchrotron radiation, the analyzer crystal of choice for the synchrotron radiation powder diffraction method is a perfect crystal of either silicon or germanium. Such perfect crystal analyzers are typically not used for traditional tube X-ray sources, because the signal rates are too low, (typically being several orders of magnitude lower than those for a synchrotron radiation experiment). The data collection times for traditional tube X-ray sources are also prohibitively long.
High resolution powder X-ray diffraction measurement at a synchrotron radiation source provides numerous advantages over traditional measurement systems using X-ray tubes. In particular, the energy of the X-rays can be freely chosen with a synchrotron radiation source; the angular resolution of this method is far superior to the X-ray tube based technique; it is possible to more accurately determine the positions of the measured diffraction peaks; one can better discriminate against fluorescence from those samples that fluoresce since only a narrow band of energies around the elastically scattered X-rays from the sample are counted, thereby reducing background interference; and background interference from air scatter in the neighborhood of the sample is also reduced. The combination of the above advantages has made powder X-ray diffraction techniques at synchrotron radiation sources a favored technique for complex materials analysis problems. In some cases powder X-ray diffraction data has made it possible to solve unknown structures of materials directly from powder diffraction data. See, e.g., F. Favier, et al., Inorganic Chemistry, 37, 1776-1780 (1998).
As mentioned above, when collecting X-ray powder diffraction data, it is possible to mount the samples in either a flat plate geometry or a capillary geometry. Both geometries have advantages and disadvantages, and it does not hold true that one geometry can be used for all samples. However, the availability of synchrotron radiation sources and the potential for using X-ray energies higher than the traditional molybdenum K-alpha radiation available from X-ray tubes has made the capillary geometry a favored geometry at synchrotron radiation sources for many analysis problems.
In a capillary geometry system, the sample is mounted in a capillary of, for example, 1 millimeter in diameter, and the energy of the X-rays is adjusted to optimally penetrate the thickness of the sample with the proper amount of absorption. In either the flat plate or capillary geometry systems, it is often necessary to rotate or rock the sample during data acquisitior. This is because the samples often do not contain an appropriate statistical representation of all possible orientations, due, for example, to preferred orientation effects, and therefore oscillating them during data acquisition can help to alleviate this problem.
A recent development in the data collection capabilities at synchrotron radiation sources is the use of multi-element detectors to measure powder patterns. See, e.g., J. L. Hodeau, et al., SPIE Proceedings, 3448, 353-361 (1998). In this high-resolution diffraction experiment performed with analyzer crystals, nine analyzer crystals and nine detectors have been mounted in parallel to detect the diffracted X-rays. Such multi-element detectors speed up the data acquisition process considerably, making it possible to rapidly analyze a sample. To further speed up the analysis of samples, it would be desirable to have a sample changer which automatically changes multiple samples mounted in a capillary geometry, while rotating the samples during data acquisition.
U.S. Pat. No. 4,770,593 to Anderson (xe2x80x9cAndersonxe2x80x9d) discloses a changer for a flat plate geometry utilizing a conventional X-ray tube. U.S. Pat. No. 4,641,329 to Green, et al. (xe2x80x9cGreen et al.xe2x80x9d) discloses a holder for a capillary geometry sample for a commercial powder X-ray diffraction instrument. These two patents address conventional X-ray diffraction with an X-ray tube source. Anderson-teaches a means for changing samples in an unattended manner in a flat plate geometry, while Green et al. teach the use of a conventional diffractometer with a capillary geometry. Neither patent teaches, nor suggests, an efficient high volume capillary geometry sample changer for use with a synchrotron X-ray source.
Creagh et al. teach a sample changer with multiple independently rotating capillary tubes for use with a synchrotron radiation powder X-ray diffractometer. See, e.g., D. C. Creagh et al., Journal of Synchrotron Radiation, 5, 823-825 (1998). Creagh et al. do not, however, teach a device for holding a vast plurality of samples for extended analysis periods. In fact, the very nature of the device taught by Creagh et al. limits the amount of samples that one can position on the sample changer before inaccuracies and alignment problems come into play. A sample changer which more effectively mounts, changes, rotates, and collects data from a plurality of samples using high resolution geometry at a synchrotron radiation source would therefore be highly desirable.
This present invention provides a sample changer for automatically changing from one sample to the next in support of high throughput X-ray powder diffraction data acquisition in a capillary geometry. The sample changer of the invention can be used with either a conventional X-ray source or a synchrotron radiation X-ray source, though the latter is preferred for high resolution measurements. During data acquisition, the samples are each rotated about the longitudinal axis of the capillary, which is aligned along the axis of rotation for the scattering angle (typically called two-theta). In a preferred embodiment, the samples are mounted on the outer rim of a disk or turntable, with each sample having its capillary axis parallel to a radius emanating from the center of the disk on which the samples are mounted. Each sample is mounted on a separate motor shaft which permits the sample to be rotated about the longitudinal axis of the capillary during data acquisition. To change from one sample to the next, the disk or turntable is rotated about its axis thereby presenting in turn each new sample to the beam. The sample changer may include switching means which begin the sample rotating about the longitudinal axis of the capillary just prior to, or concurrent with, the sample being exposed to the X-ray beam. When the data acquisition is completed for that sample, a new sample is rotated into the beam and its data collection is carried out.
In another embodiment of the invention, samples are mounted on the edge of a rectangular platform. The axis of each sample is perpendicular to the edge of the platform and parallel to the other samples. Samples are changed by translating the platform linearly parallel to the edge on which the samples are mounted, thereby presenting in turn each new sample to the beam. Like the previously described embodiment, the samples may be continually rotated about the longitudinal axis of the capillary in which they are housed, or they may be activated to rotate just prior to, or concurrent with, exposure to the beam. When the data acquisition is completed for that sample a new sample is translated into the beam and its data collection is carried out.